In a photocoupler type insulating device for transmitting analog signals, an analog input signal is A/D converted, and digital data are transmitted in the insulated section. That is, by using one LED (light emitting diode), digital data and a clock signal are modulated and transmitted. In the receiving circuit, the clock signal is regenerated from the modulated signal, and the digital data are demodulated. Such a photocoupler type insulating device can transmit high-accuracy analog signals with e.g. the input/output circuit electrically insulated. Thus, in industrial equipment operated at high voltage, a high-accuracy control signal can be correctly transmitted and received.
Pulse width modulation (PWM) can be used as a modulation scheme for the photocoupler type insulating device. In this case, a delay-locked loop (DLL) circuit can be used to regenerate the clock signal. Polyphase clocks generated by the DLL circuit can be used to demodulate the digital data.
However, the PWM signal including multibit information in one period requires a very large number of polyphase clock signals to demodulate the digital data. Thus, in the receiving circuit in which a delay-locked loop circuit and a demodulation circuit are integrated, the circuit area and the power consumption are increased. This may make it difficult to reduce the size and power consumption of the circuit.